1. Field of the Invention
This invention relates to percussive tools such as hammer drills and pneumatic hammers, and more particularly to a percussive tool incorporating a mechanism for protecting the structure of the tool against idle percussive action during no load operation.
2. Description of the Prior Art
In some forms of percussive tool, hammer blows are imparted to a tool bit by a striker reciprocated by a piston through an air cushion formed between the striker and the piston. When the tool is operated with a bit positioned in the operating position, the energy of an impact may be absorbed by the material being acted on. However, during idle percussive action in which the bit is lifted from the material, or when the bit is not mounted to the tool, the striker will strike a blow upon associated retaining members. The prior art discloses a variety of devices for absorbing the energy of an impact of the striker, thereby preventing any possible damage to the tool body.
One such prior art system has employed buffer members such as springs and rubbers for absorbing the impact energy of the striker acting on the retaining members. In tools having a relatively great impact energy, however, such intensive impact energy may be repeatedly and yet continuously applied to the retaining members, which will quite often cause damage to the retaining members. Also, the striker is likely to rebound due to the repulsive action of the buffer and hence, percussive action will be repeated, which will also cause serious damage to the structure of the tool.
U.S. Pat. No. 4,183,414 discloses a percussive tool in which a resilient retaining ring referred to as a catching collar is provided to absorb idle blows of the striker. However, it is found that the retaining ring is continually subject to shocks, and within a very short time the retaining ring will be exposed to considerable damage.
U.S. Pat. No. 3,921,729 discloses a pneumatic hammer in which a striking member driven by an air cushion produced in the bore in a cylinder, has a tapered surface which frictionally engages a tapered surface of a retainer ring, thereby limiting the striking member in its rearward movement and preventing idle blows of the striking member It is found, however, that the engagement between the two tapered surfaces is sometimes insecure. Such irregular engagement will cause the striking member to impart its impact energy to the tool and thus, the structure of the tool will be exposed considerable wear or severe damage. Also, the tapered surfaces may be strongly retained to the extent that they are hardly released from such retained position, thereby reducing the efficiency of operation.